As a conventional technology for embedding and detecting watermark information, there is a method of embedding information making use of the fact that, on condition that pixel values are distributed at random, a sum of differences converges to 0 when two points are selected spatially at random and a difference between the two points is calculated (see, for example, Non-Patent Document 1).
Non Patent Document 1: W. Bender et. al, “Techniques for data hiding”, IBM systems journal, vol. 35, no. 3-4, pp. 313-336, 1996.